Aggrecan is a major extracellular component of articular cartilage. It is a proteoglycan responsible for providing cartilage with its mechanical properties of compressibility and elasticity. The loss of aggrecan has been implicated in the degradation of articular cartilage in arthritic diseases. Osteoarthritis is a debilitating disease which affects at least 30 million Americans (MacLean et al., J Rheumatol 25:2213–8 (1998)). Osteoarthritis can severely reduce quality of life due to degradation of articular cartilage and the resulting chronic pain. An early and important characteristic of the osteoarthritic process is loss of aggrecan from the HENDERSON extracellular matrix (Brandt and Mankin, “Pathogenesis of Osteoarthritis,” Textbook of Rheumatology, WB Saunders Company, Philadelphia, Pa., at 1355–1373 (1993)). The large, sugar-containing portion of aggrecan is thereby lost from the extra-cellular matrix, resulting in deficiencies in the biomechanical characteristics of the cartilage.
A proteolytic activity termed “aggrecanase” is thought to be responsible for the cleavage of aggrecan thereby having a role in cartilage degradation associated with osteoarthritis and inflammatory joint disease. Work has been conducted to identify the enzyme responsible for the degradation of aggrecan in human osteoarthritic cartilage. Two enzymatic cleavage sites have been identified within the interglobular domain of aggrecan. One (Asn34-Phe342) is observed to be cleaved by several known metalloproteases. Flannery et al., J Biol Chem, 267:1008–14 (1992); Fosang et al., Biochemical J., 304:347–351 (1994). The aggrecan fragment found in human synovial fluid, and generated by IL-1 induced cartilage aggrecan cleavage is at the Glu373-Ala374 bond (Sandy, et al., J Clin Invest, 69:1512–1516 (1992); Lohmander et al., Arthritis Rheum 36:1214–1222 (1993); Sandy et al., J Biol Chem., 266:8683–8685 (1991)), indicating that none of the known enzymes are responsible for aggrecan cleavage in vivo.
Recently, identification of two enzymes, aggrecanase-1 (ADAMTS 4) and aggrecanase-2 (ADAMTS-11) within the “Disintegrin-like and Metalloprotease with Thrombospondin type 1 motif” (ADAM-TS) family have been identified which are synthesized by IL-1 stimulated cartilage and cleave aggrecan at the appropriate site (Tortorella, et al., Science, 284:1664–6 (1999); Abbaszade et al., J Biol Chem, 274: 23443–23450 (1999)). It is possible that these enzymes could be synthesized by osteoarthritic human articular cartilage. It is also contemplated that there are other, related enzymes in the ADAM-TS family which are capable of cleaving aggrecan at the Glu373-Ala374 bond and could contribute to aggrecan cleavage in osteoarthritis. There is a need to identify other aggrecanase enzymes and determine ways to block their activity.